Recently, an electronic technique called as “spintronics” has been brought into the spotlight. While the conventional electronics uses only “electric charge” that is one property of an electron, the spintronics positively uses “spin” that is another property of an electron in addition to that. Particularly, “spin-current” that is flow of spin angular momentum of electronics is an important concept. Since energy dissipation of spin-current is small, there is a possibility that using spin-current can accomplish highly efficient information transfer. Accordingly, generation, detection, and control of spin-current is an important theme.
For example, there is known a phenomenon that when an electric current flows, spin-current is generated. This is called as “spin-Hall effect”. As a phenomenon opposite thereto, it is known that when spin-current occurs, electromotive force is generated. This is called as “inverse spin-Hall effect”. By using the inverse spin-Hall effect, spin-current can be detected. The spin-Hall effect and the inverse spin-Hall effect are significantly exhibited in a material (e.g., Pt or Au) whose “spin orbit coupling” is strong.
By recent research, existence of “spin-Seebeck effect” in a magnetic material has been made clear. The spin-Seebeck effect is a phenomenon that when temperature gradient is applied to a magnetic material, spin-current is induced in a direction parallel with the temperature gradient (e.g., refer to Patent Literature 1, Non-patent Literature 1, and Non-patent Literature 2). In other words, by the spin-Seebeck effect, heat is converted into spin-current (heat spin-current conversion). Patent Literature 1 reports the spin-Seebeck effect in a NiFe film that is a ferromagnetic metal. Non-patent Literatures 1 and 2 report the spin-Seebeck effect at an interface between a metal film and a magnetic insulator such as yttrium iron garnet (YIG, Y3Fe5O12).
Spin-current induced by temperature gradient can be converted into an electric field (electric current, voltage) by using the above-mentioned inverse spin-Hall effect. Namely, using both of the spin-Seebeck effect and the inverse spin-Hall effect enables “thermoelectric conversion” that converts temperature gradient into electricity.
FIG. 1 illustrates a configuration of a thermoelectric conversion element disclosed in Patent Literature 1. On a sapphire substrate 101, a heat spin-current conversion unit 102 is formed. The heat spin-current conversion unit 102 has a laminated structure of a Ta film 103, a PdPtMn film 104, and a NiFe film 105. The NiFe film 105 has magnetization in an in-plane direction. On the NiFe film 105, a Pt electrode 106 is formed. Both ends of the Pt electrode 106 are connected to terminals 107-1 and 107-2, respectively.
In the thus-configured thermoelectric conversion element, the NiFe film 105 plays a role of generating spin-current from temperature gradient by the spin-Seebeck effect, and the Pt electrode 106 plays a role of generating electromotive force from the spin-current by the inverse spin-Hall effect. Concretely, when temperature gradient is applied in the in-plane direction of the NiFe film 105, the spin-current is generated in a direction parallel with the temperature gradient by the spin-Seebeck effect. Then, the spin-current flows from the NiFe film 105 into the Pt electrode 106. Alternatively, the spin-current flows out of the Pt electrode 106 to the NiFe film 105. In the Pt electrode 106, by the inverse spin-Hall effect, the electromotive force is generated in a direction perpendicular to the spin-current direction and the NiFe magnetization direction. The electromotive force can be brought out from the terminals 107-1 and 107-2 provided at the both ends of the Pt electrode 106.